1. Field of the Invention
The invention relates to modified cationic photoinitiators possessing a reduced crystallization tendency, to a process for reducing the crystallization tendency of cationic photoinitiators, and to the use of the photoinitiators of the invention for the radiation curing of cationically curing compositions.
2. Description of the Related Art
Cationic photopolymerization is a rapid, efficient and environment-friendly means of curing cationically polymerizable monomers. Particularly efficient photoinitiators are diaryliodonium salts (I) and triarylsulfonium salts (II).
Arxe2x80x94I+xe2x80x94Ar MXnxe2x88x92xe2x80x83xe2x80x83(I)
xe2x80x83MXnxe2x88x92=BF4xe2x88x92, PF6xe2x88x92, AsF6xe2x88x92, SbF6xe2x88x92
Diaryliodonium salts (I) in particular are known from the patent literature (DE-A-25 186 39, U.S. Pat. No. 4,279,717, EP-A-0 334 056, and EP-B-0 618 919) as well as their use as photoinitiators for polymerizing cationically polymerizable substances. The cationically polymerizable substances have, however, little or no polarity, especially if the polymerizable groups are present in organopolysiloxanes. When adding these photoinitiators, therefore, it is a very common observation that, depending on the structure of the formulation, the miscibility and solubility of the photoinitiators is limited. For this reason, the aryl radicals of such onium salts are often substituted with alkyl chains in order to increase the solubility in organopolysiloxanes (U.S. Pat. No. 4,310,469 and U.S. Pat. No. 4,374,066).
Diaryliodonium salts as described in U.S. Pat. No. 5,073,643 are likewise hydrophobically modified by alkyl chains. In addition, however, they carry a hydroxyl group. This results in deficient miscibility with organopolysiloxanes. After just a short time, turbidity and the precipitation of the photoinitiator are observed. Because of their inhomogeneity, such coatings do not cure very well under UV irradiation. It is also possible, however, for massive surface defects (craters, wrinkling, specks, etc.) to appear during the application of a thin layer on a substrate, as a result of the inhomogeneity.
In the case of hydroxyl-bearing iodonium salts as described in U.S. Pat. No. 5,073,643, the poor solubility in nonpolar media is attributed to the high crystallization tendency. The particular complexation characteristics of the hydroxyl-bearing iodonium salts of the general formula (III) results in a strong crystallization tendency in the compounds (A. Kunze, U. Mxc3xcller, K. Tittes, J. P. Fonassier, F. Morlet-Savary, J. Photochemistry and Photobiology A: Chemistry, 110, 115-122 (1997)): 
The two oxygen atoms in the molecule act as ligands for a second iodonium ion. This aggregation behavior promotes the formation of crystals.
In the preparation of these iodonium salts, the strong crystallization tendency is entirely desirable, since it means that the compounds can be recovered as powders in a high purity by simple recrystallization. In this way, they are easy and cost effective to prepare. Such iodonium salts are commercially available under the trade name CD-1012 from Sartomer.
The high crystallization tendency has an adverse effect, however, when these iodonium salts are to be dissolved in nonpolar media, such as organopolysiloxanes. In this case either they are insoluble or a solid precipitate forms after just a short time.
An object of the invention is to modify hydroxyl-bearing iodonium salts in a particularly cost effective and simple manner such that the crystallization tendency is greatly reduced and good compatibility with organopolysiloxanes containing epoxy groups is established.
The above object is achieved in a first embodiment by means of cationic photoinitiators of the general formula IV
[R1xe2x80x94Ixe2x80x94R2]+ Xxe2x88x92xe2x80x83xe2x80x83(IV)
where I is the element iodine,
Xxe2x88x92 is the anion of a complex metal salt or of a strong acid,
R1 is a radical 
in which Ar is a monovalent aromatic hydrocarbon radical, preferably having 6 to 14 carbon atoms per radical, or is a monovalent aromatic hydrocarbon radical containing at least one oxygen and/or sulfur atom and, preferably, having 5 to 15 ring atoms per radical,
a is 1, 2 or 3,
b is 0, 1 or 2,
c is 0, 1 or 2,
D, E and F are each substituents of Ar,
D being a radical of the formula 
xe2x80x83where
x is 0 or 1,
y is 0 or 1,
R is a linear or branched divalent hydrocarbon radical, preferably having 1 to 40 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom and/or one sulfur atom and/or one carboxyl group,
Ra, Rb and Rc each independently are selected from the group consisting of monovalent alkyl, aryl, haloalkyl and alkoxy radicals having 1 to 40 carbon atoms, preferably having 1 to 14 carbon atoms, and most preferably having 1 to 4 carbon atoms,
E is a radical
xe2x80x94Oxe2x80x94Rd,
F is a radical
xe2x80x94Re,
R2 is a radical

xe2x80x83where
Rd is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom,
Re is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom,
f is 0, 1 or 2, and
g is 0, 1 or 2.
Surprisingly, it has been found that the modification of hydroxyl-containing cationic photoinitiators makes it possible to reduce considerably the crystallization tendency and to improve substantially the compatibility with organo-polysiloxanes containing epoxy groups.
Preferred examples of aromatic hydrocarbon radicals Ar are the phenyl, naphthyl and anthryl radicals.
Preferred examples of aromatic hydrocarbon radicals Ar containing at least one oxygen and/or sulfur atom are the 2-furyl, 3-furyl, 2-thienyl and 3-thienyl radicals.
Preferred examples of the divalent hydrocarbon radicals R which can be interrupted by at least one oxygen atom and/or one sulfur atom and/or one carboxyl group are xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, and xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94.
Preferred examples of hydrocarbon radicals Ra, Rb, Rc are alkyl radicals, such as the methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl, and tert-pentyl radical; hexyl radicals, such as the n-hexyl radical; heptyl radicals, such as the n-heptyl radical; octyl radicals, such as the n-octyl radical and isooctyl radicals, such as the 2,2,4-trimethylpentyl radical; nonyl radicals, such as the n-nonyl radical; decyl radicals, such as the n-decyl radical; dodecyl radicals, such as the n-dodecyl radical, and octadecyl radicals, such as the n-octadecyl radical; and aryl radicals such as the phenyl, naphthyl and anthryl radicals.
Preferably, all three radicals Ra, Rb, Rc attached to the silicon atom together contain 3 to 25 carbon atoms.
The examples given above of the radicals Ra, Rb and Rc also apply totally to the radicals Rd and Re.
Preferred examples of hydrocarbon radicals Rd and Re interrupted by at least one oxygen atom and/or one sulfur atom are xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH3, xe2x80x94CH2xe2x80x94CH2Oxe2x80x94CH2CH3, and xe2x80x94CH2xe2x80x94CH2xe2x80x94Sxe2x80x94CH3.
Preferred examples of radicals D are
xe2x80x94O(CH2)2xe2x80x94Oxe2x80x94Si(CH3)3 
xe2x80x94O(CH2)2xe2x80x94Oxe2x80x94Si(CH2CH3)3 
xe2x80x94O(CH2)2xe2x80x94Oxe2x80x94SiMe2Oct
xe2x80x94Oxe2x80x94(CH2)2xe2x80x94Oxe2x80x94SiMe2Ph
xe2x80x94Oxe2x80x94(CH2)2xe2x80x94Oxe2x80x94SiMePh2
(in which Me is methyl radical, Et is ethyl radical, Ph is phenyl radical, and Oct is n-octyl radical).
Preferred examples of radicals E are the methoxy, ethoxy and n-butoxy radicals.
Preferred examples of radicals F are the methyl, ethyl, propyl, 2-methylpropyl and n-butyl radicals.
Preferred examples of radicals R1 are 
(in which Me, Et, Ph and Oct are each as defined above).
Preferred examples of radicals R2 are the phenyl, 4-methylphenyl, 3-methoxyphenyl and 4-methoxyphenyl radical.
Preferred examples of anions Xxe2x88x92 of a complex metal salt or of a strong acid are tosylate, SbF6xe2x88x92, PF6xe2x88x92, BF4xe2x88x92, F3CSO3xe2x88x92, F3CCO2xe2x88x92, AsF6xe2x88x92, ClO4xe2x88x92, HSO4xe2x88x92. For the purposes of the present strong acids embrace, in particular, strong Bronsted acids.
Preferred cationic photoinitiators of reduced crystallization tendency are those of the general formula V 
where D and Xxe2x88x92 are as defined above.
Particularly preferred cationic photoinitiators of reduced crystallization tendency are those of the formula VI below 
The crystallization tendency of the cationic photoinitiators modified in this way is substantially reduced in relation to the abovementioned prior art. For example, at room temperature this iodonium salt is a viscous liquid, whereas the comparable hydroxyl-containing iodonium salt having the formula (VII) below 
is a powder having a melting point of 91xc2x0 C., which is available under the trade name CD-1012 from Sartomer.
Likewise, the solubility of the cationic photoinitiators of the invention in nonpolar media, such as n-alkanes or siloxanes, is substantially greater than that of the comparable hydroxyl-containing cationic photoinitiators.
For example, the iodonium salt of the general formula (VI) has unrestricted solubility in white spirit. In contrast, the comparable hydroxyl-containing iodonium salt of the general formula (VII) is insoluble in white spirit and shows no miscibility with organopolysiloxanes containing epoxy groups.
The invention additionally provides a process for preparing the cationic photoinitiators having reduced crystallization tendency. Cationic photoinitiators of the invention are readily prepared by a diverse range of routes, as indicated below. A hydroxyl-containing cationic photoinitiator can be reacted, for example, with a chlorosilane, a chlorosiloxane, a silanol, a silyl vinyl ether, a silazane, or a cyclic siloxane.
xe2x80x9cSixe2x80x9dxe2x80x94OR+Photoxe2x80x94OHxe2x86x92xe2x80x9cSixe2x80x9dxe2x80x94OPhoto
xe2x80x9cSixe2x80x9dxe2x80x94Cl+Photoxe2x80x94OHxe2x86x92xe2x80x9cSixe2x80x9dxe2x80x94OPhoto
xe2x80x9cSixe2x80x9dxe2x80x94OH+Photoxe2x80x94OHxe2x86x92xe2x80x9cSixe2x80x9dxe2x80x94OPhoto
xe2x80x9cSixe2x80x9dxe2x80x94OCHxe2x95x90CHR+Photoxe2x80x94OHxe2x86x92xe2x80x9cSixe2x80x9dxe2x80x94OPhoto
xe2x80x9cSixe2x80x9dxe2x80x94NR2+Photoxe2x80x94OHxe2x86x92xe2x80x9cSixe2x80x9dxe2x80x94OPhoto
(xe2x80x9cSixe2x80x9dxe2x80x94O)(cyclic)+Photoxe2x80x94OHxe2x86x92xe2x80x9cSixe2x80x9dxe2x80x94OPhoto
where xe2x80x9cSixe2x80x9d denotes a general silane radical and xe2x80x9cPhotoxe2x80x9d denotes an unmodified photoinitiator radical.
The process of the invention comprises reacting a hydroxyl-containing photoinitiator having the general formula (IVxe2x80x2)
[R1xe2x80x94Ixe2x80x94R2]+ Xxe2x88x92xe2x80x83xe2x80x83(IVxe2x80x2)
in which I is the element iodine,
Xxe2x88x92 is the anion of a complex metal salt or of a strong acid,
R1 is a radical 
xe2x80x83in which Ar is a monovalent aromatic hydrocarbon radical, preferably having 6 to 14 carbon atoms per radical, or is a monovalent aromatic hydrocarbon radical containing at least one oxygen and/or sulfur atom and, preferably, having 5 to 15 ring atoms per radical,
a is 1, 2 or 3,
b is 0, 1 or 2,
c is 0, 1 or 2,
Dxe2x80x2, E and F are each substituents of Ar,
Dxe2x80x2 being a radical of the formula
xe2x80x94(O)xxe2x80x94(R)yxe2x80x94OH
xe2x80x83where
x is 0 or 1,
y is 0 or 1,
R is a linear or branched divalent hydrocarbon radical, preferably having 1 to 40 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom and/or one sulfur atom and/or one carboxyl group,
E is a radical
xe2x80x94Oxe2x80x94Rd,
F is a radical
xe2x80x83xe2x80x94Re,
R2 is a radical 
xe2x80x83where
C, E and F are as defined above,
Rd is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom,
Re is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom,
f is 0, 1 or 2, and
g is 0, 1 or 2,
xe2x80x83with a chlorosilane, a silanol, a silylamine, a silyl vinyl ether, a silazane and/or a cyclic siloxane.
Preferred hydroxyl-containing cationic photoinitiators are those of the general formula (Vxe2x80x2) 
where Dxe2x80x2 and Xxe2x88x92 are as defined above.
Particularly preferred starting materials are iodonium salts of the formula (VIxe2x80x2) below 
Examples of chlorosilanes which can be reacted with hydroxyl-bearing cationic photoinitiators are those having the general formula (VIII) 
where Ra, Rb, Rc are each selected independently and are as defined above.
Particularly preferred chlorosilanes which can be reacted with hydroxyl-bearing cationic photoinitiators are chlorotrimethylsilane, (chloromethyl)dimethylchlorosilane, (bromomethyl)dimethylchlorosilane, dimethylethylchlorosilane, triethylchlorosilane, dimethylpropylchlorosilane, tripropylchlorosilane, isopropyldimethylchlorosilane, triisopropylchlorosilane, 3-(chloropropyl)dimethylchloro-silane, dimethyl-(3,3,3-trifluoropropyl)chlorosilane, tert-butyldimethylchlorosilane, tert-butyldiphenylchlorosilane, butyldimethylchlorosilane, tributylchlorosilane, triiso-butylchlorosilane, dimethyl-(3,3,4,4,5,5,6,6,6-nonafluoro-hexyl)chlorosilane, hexyldimethylchlorosilane, dimethyloctylchlorosilane, diisopropyloctylchlorosilane, decyl-dimethylchlorosilane, dimethyldodecylchlorosilane, dimethyl-octadecylchlorosilane, dimethylphenylchlorosilane, diphenylmethylchlorosilane, triphenylchlorosilane, (pentafluorophenyl)-dimethylchlorosilane, tribenzylchlorosilane, dimethyl(2-phenylethyl)chlorosilane, dimethylvinylchlorosilane and allyldimethylchlorosilane.
Examples of silazanes which can be reacted with hydroxyl-bearing cationic photoinitiators are those having the general formula (IX) 
where Ra, Rb, and Rc are each selected independently and are as defined above.
Particularly preferred silazanes which can be reacted with hydroxyl-bearing cationic photoinitiators are hexamethyl-disilazane, 1,3-bis(chloromethyl)-1,1,3,3-tetramethyl-disilazane, hexaethyldisilazane, 1,3-dipropyl1,1,3,3-tetra-methyldisilazane, 1,3-dioctyl-1,1,3,3-tetramethyldisilazane, 1,3-diphenyl-1,1,3,3-tetramethyldi-silazane, 1,3-dimethyl-1,1,3,3-tetraphenyldisilazane, and 1,3-divinyl-1,1,3,3-tetramethyldisilazane.
Examples of silylamines are those having the general formula (X) 
where Ra, Rb, Rc, Rf and Rg are each selected independently and Rf and Rg are as defined above for Ra.
Particularly preferred silylamines which can be reacted with hydroxyl-bearing cationic photoinitiators are N,N-dimethyltrimethylsilylamine, N,N-bis(trimethylsilyl)methyl-amine, N,N-diethyltrimethylsilylamine, N-benzyltrimethyl-silylamine, 1-(trimethylsilyl)imidazole, 4-(trimethyl-silyl)morpholine, 1-(trimethylsilyl)pyrrolidine, N-(tert-butyldimethylsilyl)dimethylamine, N-tert-butyltrimethyl-silylamine, 1-(tert-butyldimethylsilyl)imidazole, N-(hexyl-dimethylsilyl)dimethylamine, N-(octyldiisopropyl-silyl)-dimethylamine, N-(octadecyldiisobutylsilyl)dimethyl-amine, N-(octadecyldiisopropylsilyl)dimethylamine, and triphenyl-silylamine.
The iodonium salts are sensitive to light and on exposure to ultraviolet light, for example, break down in accordance with a multistage mechanism which is described in the book xe2x80x9cUV Curing: Science and Technologyxe2x80x9d by P. Pappas on p. 34. The active end product of this photolysis is regarded as being the resulting Brxc3x6nsted acidxe2x80x94for example, HPF6, HAsF6xe2x80x94which in turn initiates the polymerization of cationically polymerizable substances, such as epoxides or vinyl ethers.
The iodonium salts of the invention are suitable as photoinitiators for the polymerization of cationically polymerizable organic substances, such as epoxides, vinyl ethers, organopolysiloxanes containing epoxy groups, organopolysiloxanes containing alkenyloxy groups, such as vinyloxy or propenyloxy groups, and olefins. Such substances are the content, for example, of U.S. Pat. No. 5,057,549, DE-A-40 02 922, and the patent documents cited at the outset.